The long term goals of this proposal are to improve the understanding of the pathogenesis and molecular basis for the increased intestinal Ca transport in genetic idiopathic hypercalciuric (IH) rats and whether these: findings are generalized to human IH. This colony of IH rats is characterized by hypercalciuria, normocalcemia, elevated duodenal Ca active transport, normal serum 1,25-dihydroxyvitamin D3 [1,25(OH)2D3] and spontaneous formation of Ca-containing kidney stones. Thus, the IH rats have many of the features of human IH. Preliminary studies demonstrate a two-fold increase in vitamin D receptor (VDR) binding content in intestine, kidney and splenic monocyte from IH rats. In addition, intestinal VDR mRNA is reduced, not increased. These data form the overall hypothesis that increased enterocyte VDR content is a phenotypic marker for genetic IH rats and that the increased VDR number modulates and amplifies the biologic action of 1,25(OH)2D3. If the increased VDR is critical for the increased intestinal Ca absorption and hypercalciuria, then the hypothesis would predict that the increased VDR is due to either prolongation of turnover of a normal or a mutant VDR; greater VDR tissue content increases vitamin D-dependent biologic functions; and, VDR is increased in all tissues that express the VDR gene. The following 3 specific aims are designed to test the overall hypothesis: 1) Explore the mechanisms whereby VDR is increased in IH rats by: vitamin D-dependence, tissue distribution and content, and developmental appearance of the VDR; identify abnormalities in the exons of the VDR gene of IH rats by PCR amplification of intestinal VDR cDNA; determine VDR message stability and VDR turnover. 2) Test the biological function of increased VDR in tissues in which the VDR is increased by: Ca active transport and VDR content in intestinal segments of high and low transport rates; calbindin 9kd gene expression; 1,25(OH)2D3-induced interleukin 1-beta (IL-1) gene suppression in splenic monocytes; osteocalcin production and 24-hydroxylase activity in calvarial bone cells. 3) With Project 3, determine VDR content and biologic function in accessible tissues from patients with IH: develop VDR measures in peripheral blood monocytes and EBV transformed lymphoblasts; and 1,25(OH)2D3-dependent suppression of IL-1 gene expression. The proposed studies should provide more detailed knowledge of the pathogenesis and molecular basis for genetic IH in the rat model and provide potentially new and useful insights into genetic human IH.